1. Field of the Invention
The present invention relates to differential locks for vehicle differentials, and particularly to the mechanism for activating the differential lock.
2. Description of the Related Art
A differential typically is provided in a vehicle to allow speed differences between driven wheels on either side of the vehicle during cornering. One problem with using a differential for this purpose is that if one of the wheels connected to the differential looses traction, drive to both wheels fails. Accordingly, the differentials of vehicles which are likely to loose traction on one of their wheels, e.g., off-road vehicles, typically are provided with a differential lock to selectively prevent relative rotation of the parts of the differential.
While there are many types of differential locks, one of the more common is a hydraulically actuated clutch which, when activated, prevents relative rotation between the housing of the differential and one of the side gears of the differential. Such clutches normally are activated by a hydraulic piston that rotates with the differential, e.g., as taught in U.S. Pat. No. 3,913,414 (Freiburger). The disadvantage to this structure is that rotating seals are required somewhere in the hydraulic system providing oil to move the piston.
Such rotating seals have several disadvantages. First, they almost always inherently leak. If the differential lock is used in an independent axle spaced away from the remainder of the transmission, some means must be devised to return the leaked oil to the reservoir from which it came, e.g., as taught in U.S. Pat. No. 4,235,307 (Browning et al.). Second, such seals generally have a maximum pressure limit of about 2700-4100 kPa (400-600 psi). Many off-road vehicles are work vehicles, e.g., tractors, bulldozers and the like, which have a typical system pressure of about 15,500 kPa (2250 psi). This means that a pressure-reducing valve must be provided to supply hydraulic fluid to operate the differential lock.
U.S. Pat. Nos. 4,644,823 (Mueller) and 4,679,463 (Ozaki et al.) partially overcome these problems by providing non-rotating pistons located in the differential housing, but at the cost of extra size in the differential housing. In addition, both of these devices use specialized differential housings, which decreases manufacturing flexibility.